The present invention relates to a filtration apparatus and, more particularly, to an improvement of a filtration apparatus in which a so-called pre-coat layer of a filter aid is formed on the surface of a filter element or filter medium and the liquid is filtered through the thus formed pre-coat layer. Filtration apparatuses of this type are widely used in the manufacturing process of alcoholic drinks such as beer, wine, Japanese sake and the like, seasonings such as soy sauce, vinegar, sugar, cooking oil and the like, other beverages such as fruit-based drinks, carbonated drinks, mineral water, Oolong tea and the like, pharmaceuticals and so on.
As is known, a type of filtration procedure utilizes a pre-coat layer which is formed of a filter aid such as diatomaceous earth, perlite, cellulosic powder, active carbon and the like on the surface of filter elements in a substantial thickness and through which the liquid is filtered.
The most conventional material as the filter element, on which the pre-coat layer is formed, includes filter cloth, filter net, filter paper and the like although several more rigid materials are sometimes used including punched metal tubes spirally wound with a wire, corrugated rectangular tubes (or tubes having an undulate polygonal cross section) spirally wound with a wire, ceramic-made cylindrical elements and so on. Different types of the filter elements may be used either alone or as a combination of two types or more.
Various types of filtration apparatuses utilize one or more cylindrical or polygonally tubular filter elements provided with a pre-coat layer in an upright disposition. These filtration apparatuses are classified into two classes depending on the position at which the filtrate is taken out of the filter element. Namely, the filtrate may be discharged at an upper part or at a lower part of the filter element. A problem in each of these filtration apparatuses is that, since the pre-coat layer is formed on a vertical surface, the pre-coat layer sometimes falls off the surface of the filter element not only in the course of forming of the pre-coat layer of a filter aid but also during the filtering operation through the thus formed pre-coat layer, especially, when the liquid flow through the pre-coat layer is interrupted.
In connection with forming the pre-coat layer of a filter aid on the surface of a filter element, it is a possible way that a dispersion of the filter aid, from which the filter aid should be deposited to form the pre-coat layer on the surface of the filter element, is prepared by use of a liquid which is the liquid to be filtered therethrough. This method, in which the filter element and the filter aid are contacted from the very beginning by the liquid to be filtered, sometimes causes problems when the amount of certain soluble materials dissolved out from the filter aid and entering the filtrate must be limited or when microbiological contamination of the filtration system must be avoided. It is a usual practice in such cases that the pre-coat layer is formed using a dispersion of the filter aid in a liquid which is not the same one as the liquid to be filtered, such as cold or hot water, and thereafter the liquid passing through the precoat layer is gradually switched to the liquid to be filtered without interrupting the liquid flow therethrough.
When a process of filtration is started with formation of a pre-coat layer in the above described manner, it is an unavoidable consequence that a considerably large portion of the liquid coming out of the filter element in the course of the above mentioned liquid switching is a mixture of the first liquid used as the dispersing medium of the filter aid to form the pre-coat layer and the second liquid which is the liquid to be subjected to the filtration treatment. Such a fraction of liquid mixture naturally contains the desired filtrate constituent only in a low concentration so that it is discarded as a waste or stored in a separate reservoir from which it is returned bit by bit to the preceding step of the process.
The above mentioned way of discarding the transitive filtrate is undesirable when the filtered liquid is very expensive or when certain detriment is anticipated by discarding the liquid while the alternative way of collecting the transitive filtrate in a separate reservoir before disposal or recycling is economically disadvantageous in respect of the investment for the facilities and consumption of large manpower and time.
In order to solve the above mentioned problem concerning the transitive filtrate which is a mixture of the liquid used for pre-coat forming and the liquid to be filtered, a method has been proposed in which the space inside the filter apparatus is pressurized with a gas such as air, carbon dioxide and the like after completion of formation of the pre-coat layer on the surface of the filter element so as to discharge the liquid used for pre-coat forming. This method also cannot provide a complete solution of the problem. For example, the pre-coat layer may fall off the surface of the filter element partly or wholly during the discharge of the liquid under pressurization or in the course of filling the filter element with the liquid to be filtered therethrough and tiny holes are sometimes formed in the pre-coat layer so that it is a difficult matter to keep the pre-coat layer in a complete defect-free condition. This problem is particularly serious in a filtration apparatus of the type in which the filtrate is taken out of the apparatus at an upper part thereof.